(1) Field of the Invention
This invention concerns landfill bioreactors and methods for their use to accelerate anaerobic and/or aerobic degradation of municipal solid waste in a manner that increases the landfill capacity.
(2) Description of the Art
The bioreactor landfill concept has been examined in lab and pilot scale projects since the 1960's (Merz). Merz found that placing refuse in thick lifts with continuous leachate spraying on the working face can provide increased landfill densities of 35%. He also discovered that a landfill constructed in an aerobic manner with leachate/water addition can increase settlement rates as much as three times that of an anaerobic landfill. In 1969 the U.S. Public Health Service (predecessor to USEPA), funded research to investigate processes that would result in maximum conversion of municipal solid waste (MSW) to gas (methane and CO2). Since energy was cheap at the time, a goal was to reduce the weight and volume (i.e., increase the density) of the solid waste remaining for disposal. However, the state of the first energy crisis started in 1973 led to projects focused on enhanced methane production. Large scale projects were conducted in the early to mid '70s by Leckie & Pacey (1979) and Ham (1982).
Profs. Robert Ham and Fred Pohland have been studying methods to enhance methane production and waste degradation for the last 30 years. Both have used measured increases in methane production and viewed methane as an energy source to be exploited as the primary goal of landfill stabilization. The DOE sponsored several projects in the late '70s during the "second energy crisis" to demonstrate that MSW could provide a renewable source of energy with bioreactor technology (Waste Tec 1986). A bench scale study conducted in Spain (MataAlvarez, 1986) showed that with optimum temperature range of 34-38.degree. C., inoculation of digested pig manure, and moisture contents of 87% by leachate recirculation, more than 90% of biodegradable matter was degraded within 25-57 days. The model for a scaled up landfill showed a landfill life of 1.5-2 years with 95% of the biodegradable matter being reduced during the first year.
In 1988, USEPA in the preamble to the draft Subtitle D rules indicated that leachate recirculation (as allowed by the draft and final Subtitle D regulations) should provide the following benefits: 1. Increases the rate of waste stabilization; 2. Improve leachate quality; 3. Increase the quantity and quality of methane gas production; 4. Provide a viable on-site leachate management method; 5. Maximize rate and quantity of methane energy recovery; and 6. Shorten the duration of methane generation and reduces long term risks.
Matsuto (1991) studied the concept of constantly maintaining an aerobic layer at the bottom of a landfill. Based on bench scale work as well as modeling, with BOD removal from leachatc was achieved in the bottom aerobic layer. This confirmed the work of Stegman (1987) who also observed that the methanogenic stage occurred much faster.
Stessel (1992) conducted unique lab studies using aerobic treatment of all MSW with leachate recycle. This work showed MSW could take up to 70% moisture (wet weight) and achieve 50% settlement. Rapid degradation of waste and leachate could occur within months. He later wrote about and provided conceptual designs of the re-usable landfill and piloted work on landfill mining techniques. No methane is generated in this process therefore no gas collection systems are required. The primary goal of the Stessel study was rapid stabilization and re-use of landfill space.
In 1995 the USEPA Office of Research and Development sponsored the first of two workshops on Landfill Bioreactor Design and Operation. The projects presented in 1995 and 1996 workshops reviewed the "application of leachate recirculating municipal solid waste landfills aimed at reducing environmental risk and optimizing environmental risk and optimizing landfill volume by encouraging active biological decomposition within the contained waste system." (USEPA, 1995). The ORD also acknowledged that EPA sponsored studies in the early '80s demonstrated that water, leachate recirculation, and sludge addition all enhanced methane production for energy recovery.
A good review of literature on bioreactors is found in two papers by Komilis, Ham, and Stegmann (1999) and in a book by Reinhart and Townsend "Landfill Bioreactor Design &Operation" 1997. An observation made in both references is that maintaining high levels of moisture evenly within the landfill with leachate recirculation is the key to rapid stabilization of leachate and waste. The authors determined that the prior lab scale work focused on leachate recirculation as the primary method of affecting leachate quality, waste stabilization, waste settlement, gas production, attenuation of heavy metals and priority pollutants, and other factors. The parameters that were evaluated were moisture content, pH, temperature, availability of macro- and micro-nutrients and the presence of suitable microorganisms as the main parameters controlling landfill stabilization. Pre-treatment techniques such as thermal, mechanical (i.e., particle size) and biological were also examined. Additives other than leachate were studied. The additives included water, anaerobically digested sludge that ensured suitable anaerobic and facultative microorganisms are present, and other liquids. Some of these studies had contradicted other studies, especially on the importance of adding buffer, nutrients, and sludge in enhancing degradation. Most studies concluded that increasing moisture distribution and content up to 70% on a wet-weight basis optimized the speed of biodegradation.
The most prominent and frequently cited case histories in the literature are: Leckie and Pacey, (1979), Lycoming County, Penn. (1978-'85), Seamer Car Landfill, UK (1979-1984), Delaware Solid Waste Authority (DSWA), numerous sites in Germany started in 1981, but most notably Bornhausen Landfill and reviewed by Stegman and Spendling (1989), Binghamton, N.Y., and SORAB, Sweden.
Leckie and Pacey conducted a demonstration on 6 large test cells at Mountain View, Calif. They found that leachate recirculation resulted in rapid stabilization of the waste as indicated by direct measurements of VS, cellulose content, carbon/nitrogen, and carbon/phosphorus ratios. They also concluded that high moisture content and sludge addition increased methane production. Settlement was also measured at 20-25 percent. Some problems were noted with gas leaks and water infiltration.
Lycoming County Landfill was one of the first operating landfills to practice leachate recirculation at full scale. A variety of recirculation methods were tried including spray irrigation, vertical wells, open trenches, and trenches filled with auto fluff or baled fiberglass. The last two methods were most effective in wicking the leachate to larger areas of the refuse. Results were improved waste degradation and methane generation, rapid stabilization of leachate quality (close to pilot-scale studies), and empirical evidence of increased settlement compared to dry areas of the landfill. Also, the importance of eliminating clayey daily cover or pushing back daily cover to allow leachate to drain was discovered.
The Seamer Car Landfill in Britain conducted a full scale demonstration of leachate recirculation by spraying leachate on top of the landfill and enhanced the method with surface furrowing. A low permeability intermediate cover, however, created a perched water table. This created a saturated condition above the base liner of the landfill, but showed that with increased moisture content, there was a more rapid reduction in leachate organic strength. Significant reductions in organics were noted within 2-3 years of operation. They also raised a concern for the residual COD, ammonia and chloride concentrations remaining in the leachate, although the metals and organics were treated.
The Delaware Solid Waste Authority (DSWA) has recirculated leachate in its three landfills since 1981. A variety of methods have been used including spray irrigation, recharge wells, and surface application. The main advantage of leachate recirculation was the avoided costs of building a leachate treatment plant estimate to cost up to $6 million. Other benefits included accelerated biodegradation of organics in the waste, reduced risks to the environment, and increased production of landfill gas.
Germany had over 13 landfills practicing leachate recirculation in 1981 using spray irrigation, spray tankers, and horizontal distribution pipes. Fast reduction in BOD and COD was reported after four years and no increase in salts or heavy metals was noted. Also, at sites that had waste placed in thin-layers (i.e., 6 ft.) leachate was observed to have very low strength. The Bornhausen Landfill incorporated both thin layer operations and leachate recirculation. This site was studied by Stegman (the Ham/Pohland equivalent of Germany). The thin layers were loosely compacted as opposed to rapid vertical filling. This promoted natural ventilation and some aerobic decomposition. Oxygen penetrated up to 3 feet within the waste mass. Three test sites were set up at the site. Leachate recirculation resulted in a 50 percent decrease in time required for stabilization in the site without lcachate recirculation (230 days vs. 460 days). Another significant application involved the introduction of highly concentrated leachate from new landfill cells over older cells in which stabilized leachate was already being produced. This showed a large increase in treatment with a 90 and 99 percent reduction in COD and BOD respectively. This showed that stabilized waste is very effective in providing additional treatment.
Nanticoke Landfill in Binghamton, N.Y. was one of the first sites to investigate the affects of leachate recirculation on enhancement of landfill gas. The leachate recirculation parameters studied were moisture content, pH, temperature, and nutrients. Nutrients were controlled by varying the quantity of wastewater treatment plant sludge added to the waste. The highest gas production was in cells with sludge (over an order of magnitude higher than cells with no sludge). Also, the best leachate quality was in cells with high gas yielding cells. The conclusion was that sludge added at a rage of 0.45 kg per 115 to 160 kg of MSW would produce optimum results for gas production, gas quality, and leachate quality.
The SORAB, Sweeden test cells used recirculated leachate that was heated to maintain a temperature of 35 to 40.degree. C. The gas production was reported to be an order of magnitude higher than typical.
Several bioremediation sites have been developed by WMI. The Spruce Ridge Resource Management Facility started recirculating leachate in July, 1997 using lateral drains over about 40 feet of waste and again on the final grade of about 90 feet of waste. Gravel backfill is used with perforated HDPE pipe. The unique features of this site demonstration were the installation of settlement plates at both an adjacent cell operated the normal "dry"method and the leachate recirculation cell. Settlement has been measured from 18 to 20 percent and recent density measurements are about 1900 lbs/yd3. Waste is originally placed at a density of 1200 to 1300 lb/yd3. Gas wells were just installed last spring and around the area of lateral drains. The waste appeared to be between dry to moist. Gas production is high quality with about 55 to 60% methane. It is estimated that gas production is 3.5 times the other areas of the landfill that were conventionally operated (i.e., dry methods). Recent leachate data from cell 2 shows substantial enhancement in degradation compared to the control.
In 1998, the Earthmovers Landfill, in Elkhart, Ind. began recirculation of almost 4 million gallons of leachate into 2 cells. Both cells have dedicated 4-inch slotted HDPE drain pipe with 1-6" tire chip filled trenches that vary from 150 to 300 feet in length. The system was designed using a modified groundwater model for spacing, flow, and distance from side slopes. Leachate is injected at a rate of 100 gpm with 6-30 minute rest periods during the day. This equals a rate of 100 gallons per foot of trench per day. The site takes about 80% industrial and special waste and 20% residential. The leachate generation rate is 56 gallons per acre per day.
The Atlantic Landfill in Waverly, Va. includes a cell having 2 levels of HDPE pipe with perforations every 10 feet to ensure lateral distribution to the end of the pipe. Tire chips were used as backfill and pipe was laid in trenches that are 50 feet apart. In the fall of '98, due to new cell 3 construction and storms, over 3.5 million gallons was injected in Cell 2. During the first 3.5 months of 1999, new surveys showed that cell 2 reclaimed over 48,000 yds with settlement of 3 to 4 feet. A force main system is used to pump in leachate with an average flow of 80 gpm for 2 weeks straight. The gas flow is estimated to be 10 times normal gas production.
The Middle Pennusisula Landfill in VA includes one cell including a 450 foot long perforated HDPE pipe with tire chip backfill in the first 40 feet of gabarge over lain by another 40 to 60 feet of garbage. This pipe handled up to 500 gpm before running out of leachate. The overburden of garbage seems to have restricted flow and it was assumed that the lower lift was saturated. Gas wells, however, recently were installed on top of the cell and drilled within feet of the injection line. (Gas also is collected from the leachate injection line on periods of rest with a valve that directs gas to the active collection system.) The garbage from the gas wells appeared dry to moist, but not saturated.
Leachate recirculation has been practiced in the Blackoak Landfill in Missouri with lateral injection lines of perforated HDPE pipe with a force main feed to a header line to horizontal trenches in one cell. This has enabled the site to avoid any off-site leachate hauling for the year of operation of the system. The system was installed as soon as the cell was 30 feet tall. An additional 20 feet of waste was placed on top of the recirculation galleries. There was a 10 foot drop in grade over the total 50 feet of height of the cell. Leachatc data from the recirculation cell was compared to the old cells that were operated normally (dry). Leachate from the recirculation cell was enriched in organics and ammonia nitrogen showing substantial increased degradation of the waste.
Phase I of a demonstration of an aerobic method of accelerating degradation of MSW was conducted at Live Oak Landfill, Atlanta, Ga. A 2.5 acre test cell with 70,000 cubic yards of waste that was three years old and fully anaerobic was used. Vertical injection wells for leachate and air were installed in the test cell. Leachate recirculation began for a month before air was injected. Air injection caused an immediate reduction of methane from 55 percent to less than 10 percent within 24 hours. Within 14 weeks of air injection, the landfill settled one foot out of 30 feet. Also, the waste from drilling tests showed a compost-like material that was stable according to respirometry tests. Additional testing showed the screened waste materials would pass compost standards of the State.
The cell continued to operate aerobically for 9.5 months and 1.8 million gallons of water and leachate were added at an average of 6,819 gallons per day. Settlement measurements over 6 months indicated about 15 percent settlement. Leachate quality peaked in organics by the fourth month of operation and started to decline when monitoring activities ceased. This type of leachate recirculation demands about 7-10 times moisture available through leachate on site.
The Springhill Landfill in Florida sprays leachate on the working face at a rate of about 10 gallons/cu yd. Densities have increased from about 1250-1300 lbs/cu yd to 1800 lbs/cu yd over the last three years.
At a site is owned by Yolo County in Woodale, Calif., a pilot landfill bioreactor and a control cell was constructed in 1996. Two lined cells were constructed and filled with 8,568 tons of solid waste and 1,336 tons of green waste as alternative daily cover while the control cell had 8,737 tons of solid waste and 1,454 tons of alternative daily cover of green waste. Both sites were built in a pyramid shape above grade and about 10 to 15 feet below grade. A geomembrane liner was used to cover both sites before liquid was added to the enhanced cell.
Liquid (groundwater and leachate) was added to the site starting on Oct. 23, 1996 up to Oct. 15, 1998 after which leachate continued to be recirculated at a rate of several hundred gallons per day. A total of 1,159,616 gallons of liquid was added to the apparent field capacity of 46% dry waste weight assuming a 20% moisture of in place waste. The waste adsorbed up to 45 gallons per ton of "as-placed" waste. As of May, 1999, the enhanced cell settled 67 inches (about 15 to 18%) compared to the control cell settlement of 10 inches (about 2%). Landfill gas volume was over twice the annual volume from the beginning of liquid addition to May, 1999 with the average flowrate of over 4 and 7 times the control cell during 1998 and up to May, 1999 with the control cell declining at a faster rate and lower yield. The bottom of the site had transducers and piezometers that have shown. Leachate data in the enhanced cell showed enrichment during the first year in organics and TDS and a decline in subsequent years. VOCs and metals followed a similar trend. The control cell yielded a very dilute leachate showing very little organic decomposition.
The county owned landfill in Worcester Co. Maryland, started leachate recirculation in 1991 in order to control leachate disposal costs. Off-site hauling of leachate was costing the county over $100,000 per year. leachate recirculation was accomplished from the start of operations using concrete manholes that were build from the first operations layer upward to final grades. The bottom ten feet was solid pipe with the upper portions perforated. Leachate was recirculated using a water truck and manholes were installed at one per acre. The site recirculated two thirds of the leachate produced and trucked the other third off-site. This continued until 1998 and leachate quality was monitored quarterly for VOCs, metals, and indicators. This is the only long term full scale site that demonstrated that leachate quality follows lab findings and theory and increase in contaminant concentration at first and then improve in quality with time. Borings and test pits showed the amount of degradation was correlated with depth, with more moisture content increasing with depth. The upper quarter section of the landfill was the least degraded, showing that vertical injection most likely is not as efficient as horizontal pipe/trench applications. The degraded waste material was trommeled and screened with the fines passing the Maryland tests for compost. The State verbally approved the use of the fines for daily cover for the new cells. It is estimated that with construction and use of compost in three additional cells, that the original cell will be 50% depleted in its original volume. The waste filling then will return to cell one and continue leachate recirculation using the composted material from cell 2 for daily cover or eventually off-site applications.
A project was conducted in the field by the University of North Dakota, at Grand Forks for 6 years. One small cell of MSW was injected with landfill leachate and recirculated. The other cell had only water injected with the subsequent leachate removed and discarded and replaced by an equal amount of clean water. The results showed that within the first year, the leachate recirculation cell had higher concentrations of the tested constituents than the water addition cell. After the first year, the levels of contaminants were equal and the relative same level of treatment was achieved in both cells for the tested analytes of COD, metals, and pH.